Spend some time removing the undercoating from the rust-damaged panel. You will be able to see the correct and original installation method used in the original installation. The front panel was originally spot welded from the bottom but also inside during factory assembly. The front firewall bottom actually bends at the bottom and forms a support ledge about 3/4" wide in the front of your new floor panel. Carefully observe where and how the original metal was welded. All the information is in front of you. You can use a heat gun and scrapper to remove undercoat. I use a small angle grinder with a tough twisted wire brush. Be sure to use some leather gloves, face shield and dust mask if you go that route. You'll find that nearly all the attachment is done by spot welds. Most shops use "plug welds" (drill a hole in first piece of metal then mig weld both metals together, grind smooth to finish). This procedure simulates the original spot welds in appearance and performance.
